Partially carbonized polymeric fibrous materials are known in the prior art and commonly are formed by the thermal processing of a polymeric fibrous material wherein the maximum carbonization temperature utilized is less than that employed for the production of true carbon fibers which contain at least 90 percent carbon by weight. For instance, a maximum carbonization temperature in a non-oxidizing atmosphere of approximately 600.degree. to 1150.degree. C. commonly is employed when forming a partially carbonized polymeric fibrous material while a maximum temperature of 1300.degree. C. or more commonly is employed when forming carbon fibers containing at least 90 percent carbon by weight. While heating in a non-oxidizing atmosphere, elements other than carbon such as oxygen substantially are evolved and a backbone of carbon atoms is formed which provides a route for electron movement. Generally the higher the maximum carbonization temperature, the lower the electrical resistivity of the resulting fibrous product in the direction of its length.
The partially carbonized polymeric fibrous materials heretofore available, while holding potential for utilization in a number of end uses, have been observed to exhibit highly unstable electrical properties when exposed to ambient conditions. Accordingly, it has been observed that the electrical resistivity of a partially carbonized polymeric material will increase significantly upon exposure to an unprotected environment (e.g., to ambient conditions). The change (i.e., increase) in electrical resistivity commonly is the greatest for those partially carbonized polymeric fibrous materials which were formed at the lower end of the temperature range heretofore specified. It has been observed that such increase in electrical resistivity upon the passage of time will still be operative after two years of aging at ambient conditions.
When the partially carbonized polymeric fibrous materials of the prior art are selected for end use applications where the electrical properties are of importance (e.g., for electrostatic charge dissipation or for electromagnetic radiation shielding), the change in resistivity over time greatly complicates inventory maintenance and the service reliability of the product. Accordingly, the change in electrical resistivity with time must be factored into the design of the product or the product must be periodically replaced when its changing electrical resistivity moves outside of the prescribed specifications for a given end use.
For a discussion of the decrease in electrical conductivity upon aging in air of partially carbonized fibers derived from acrylic fibers see "Electrical Conductivity and Electro-Spin Resonance in Oxidatively Stabilized Polyacrylonitrile Subjected to Elevated Temperature," by N. R. Lerner, J. Appl. Phys. 52(11), November 1981, pages 6757 to 6762.
It is an object of the present invention to provide an improved process for the production of a partially carbonized polymeric fibrous material having an electrical resistivity of enhanced stability.
It is an object of the present invention to provide an improved partially carbonized polymeric fibrous material which exhibits an electrical resistivity of enhanced stability.
It is another object of the present invention to provide a process for adjusting the electrical resistivity of a partially carbonized polymeric fibrous material to a value which thereafter exhibits an enhanced electrical stability when compared to a similarly prepared fibrous material of substantially the same electrical resistivity which was not subject to step (b) of the present process.
It is a further object of the present invention to provide an improved partially carbonized polymeric fibrous material which particularly is suited for use in applications involving electrostatic charge dissipation or shielding for electromagnetic radiation.
These and other objects, as well as the scope, nature, and utilization of the claimed invention will be apparent to those skilled in the art from the following detailed description and appended claims.